Add PasswordUnlockViewController mock instead of an alert view.

Following 1Password's UI, added a full-screen password unlock view controller. It's just mock UI, but it is functional and seems to be a bit nicer than presenting an alert view to the user.

![password_unlock](https://f.cloud.github.com/assets/497951/1824364/734cce80-7195-11e3-9d50-2ef41158c57e.png)

![1pass_lock_screen](https://f.cloud.github.com/assets/497951/1824369/9d6311ac-7195-11e3-9301-a1dac60f067a.png)